Often it may be useful to separate different compounds in a mixed sample. In order to do so, an analytical chemist may use chromatography. There are several different types of chromatography, which may be preferred for different types of samples. Some of these types of chromatography include carbon dioxide as the mobile phase. One example of these types of chromatography is Supercritical Fluid Chromatography (SFC).
Frequently, after a separation performed by chromatography, the separated analytes need further analysis by a mass spectrometer, to give structural information about the analytes. However, several types of chromatography systems, including SFC systems, operate at high pressures and high temperatures, which can make the eluent difficult to efficiently interface with mass spectrometers, or other analytical instruments for further analysis. The high pressure is often maintained by a back pressure regulator (BPR). However, introducing the mobile phase flow to the mass spectrometer after passing through a BPR is ill-advised since the volume of the back pressure regulator can lead to a reduction in the chromatographic efficiency (i.e. lead to a reduction in the sharpness of the chromatographic peaks). Further, since there is no appreciable mobile phase density after exiting the BPR and because analyte solubility is directly related to mobile phase density, analyte transport to the mass spectrometer becomes problematic. Also, BPRs are not effective when operating with the low mobile phase flow rates encountered with microfluidic chromatography applications.
A further problem to interfacing SFC, and some other chromatography systems, to mass spectrometers is where the mobile phase in the chromatography is carbon dioxide. When only carbon dioxide comprises the mobile phase, analytes present are not easily ionized in the electrospray ion source of a mass spectrometer. Often, this is solved by adding a make-up fluid (i.e. methanol) to the mobile phase downstream of the column before it is introduced to the mass spectrometer. This involves more plumbing, which may also lead to the sharpness of the peaks from the chromatography system to be reduced, and can create extra cost to the instrument (i.e. require an additional high-pressure pump).
There is, therefore, a need for an improved interface between a carbon dioxide based chromatography device and a mass spectrometer. There is also a need for an alternative apparatus for interfacing a carbon-dioxide based chromatography device and a mass spectrometer that overcomes or at least mitigates the problems associated with the prior art apparatus and systems.